1. Field of the Invention
The present invention relates to a motor control device for controlling a plurality of motors that drive one driven body moving along one movement shaft.
2. Description of the Related Art
In a machine tool, a robot, or the like, when at least one motor that drives one driven body moving along one movement shaft (e.g., a gravity shaft along which the driven body may moves due to gravity) is controlled, an abnormality of the motor is detected by for example temperature. Thereby, an alarm is generated, and the state of the brake that brakes the driven body is switched from a releasing state to a fastening state when a brake operation completing period of time set in advance lapses from the alarm generation timing.
When the alarm is thus generated, the motor cannot be normally controlled. Accordingly, the state of the motor needs to be switched from an excited state (driven state) to an un-excited state (un-driven state). For this purpose, the following motor control devices are proposed, for example. The first motor driving device switches the state of the motor from the excited state to the un-excited state at the first timing when an alarm occurs, as described in Japanese Patent No. 2954616, for example. The second motor control device switches the state of the motor from the excited state to the un-excited state at the second timing when the brake operation completing period of time set in advance lapses from the alarm generation timing, as described in Japanese Patent No. 2898288, for example.
Another motor control device is proposed. This motor control device controls a plurality of motors to drive the driven body by a plurality of the motors, for example, because the a size of the driven body is large so that acceleration and deceleration are difficult for one motor due to insufficient torque, as described in Japanese Laid-open Patent Publication No. H7-110714.
When one driven body is driven by a plurality of motors, generation of an alarm for one motor among a plurality of the motors prevents normal control of the motor that has caused the alarm. Accordingly, the state of the motor that has caused the alarm needs to be switched from the excited state to the un-excited state at the above-mentioned first timing, and states of the motors other than the motor that has caused the alarm need to be switched from the excited states to the un-excited states at either the above-mentioned first timing or the above-mentioned second timing.
However, according to the conventional motor control device that controls a plurality of motors for driving one driven body by these motors, the timing when states of the motors other than the motor that has caused the alarm are switched from the excited states to the un-excited states is not set on the basis of a physical quantity concerning at least one of the motor and the driven body. For this reason, at least one of the motor and a system (e.g., a feed screw mechanism including a movement shaft) associated with the motor possibly suffers an adverse effect.
For example, when controlling a plurality of motors that drive one driven body moving along one gravity shaft, when an alarm for one motor among a plurality of the motors is generated while the driven body is stationary, switching states of the motors other than the motor that has caused the alarm, from the excited states to the un-excited states at the above-mentioned first timing causes the driven body to drop before the brake operation completing period of time set in advance lapses from the alarm generation timing. As a result, the driven body possibly interferes with another object (e.g., a workpiece arranged on a table of a machine tool).
When controlling a plurality of motors that drive one driven body moving along one movement shaft, when an alarm for one motor among a plurality of the motors is generated, switching states of the motors other than the motor that has caused the alarm, from the excited states to the un-excited states at the above-mentioned second timing allows torque of the motors other than the motor that has caused the alarm, to be generated while torque of the motor that has caused the alarm is not generated. To bring acceleration of the driven body after the alarm generation to the same acceleration of the driven body before the alarm generation, magnitude of torque generated by the motors other than the motor that has caused the alarm needs to be increased after the alarm generation so as to be larger than magnitude of torque generated before the alarm generation. For example, when one driven body is driven by two motors, the motor other than the motor that has caused the alarm needs torque of which magnitude is approximately twice as large as torque generated before the alarm generation. Thus increasing magnitude of torque leads to increase in magnitude of loads of the motor and the system including the motor. As a result, the motor and the system including the motor may suffer an adverse effect.